KABUL (Reuters) - European observers urged Afghanistan to deepen its investigation into vote-rigging on Thursday as candidates held closed-door meetings to try to resolve a deadlock threatening to split the country along ethnic lines.

Former resistance fighter Abdullah Abdullah and ex-World Bank economist Ashraf Ghani have locked horns over the outcome of the June 14 run-off election, with both claiming victory in the contest to succeed Hamid Karzai as president.

Intensifying speculation about a possible backroom deal to resolve the impasse, preliminary results have been delayed until next week after the Independent Election Commission said it would recount votes from 1,930 polling stations.

On Thursday, observers from the European Union said the investigation should be broadened and other aspects should be examined such as an improbably high number of votes for one candidate at some polling stations.

"If you would use these factors as well and investigate all polling stations ... on the basis of these factors you may well end up concluding that over 6,000 polling stations in the country need a thorough investigation," EU Election Assessment Team Chief Observer Thijs Berman told reporters.

"This is a worryingly high figure of around 28 percent of polling stations throughout the country. I have grave concerns on these high figures and insist on the necessity to enlarge the audit as these figures are so high."

The protracted dispute over the credibility of the election has all but destroyed hopes of a smooth transition of power in Afghanistan, its security outlook already weakened by the steady withdrawal of most foreign troops throughout this year.

Behind closed doors in the capital Kabul, representatives of both camps have been holding negotiations to seek a political solution - possibly a power-sharing agreement that would see both men assume top roles in post-Karzai Afghanistan.

Little has come out of these meetings, but one senior official said the two camps appeared to have made some headway, raising the possibility of a conciliatory direct meeting between Abdullah and Ghani.

DANGEROUS RIFT

Abdullah draws his support from the minority Tajik community while Ghani represents the Pashtun majority, and Kabul has been abuzz with speculation about a broader rift along ethnic lines or more violence unless the standoff is resolved soon.

Ghani's aides, citing election observers, say he is in the lead in the run-off by one million votes. But official figures will not be released until Monday.

Abdullah has accused Karzai, an ethnic Pashtun, of playing a role in the alleged rigging in Ghani's favour, and last week thousands of Abdullah's supporters marched on the presidential palace in a peaceful protest.

Chief electoral officer Zia ul-Haq Amarkhail has since resigned but denied any role in the suspected fraud.

Speaking to Afghanistan's Tolo News, IEC chairman Ahmad Yousuf Nuristani said the vote recount would not affect the outcome of the election and it was not immediately clear if the IEC would heed the EU observers' recommendation.

"The total number of votes from 1,930 polling stations going to be audited is 1.1 million. I think this will not affect the outcome but obviously unclean votes will be thrown out," he said.

A messy vote recount would exacerbate anxieties in Kabul and for the United States, which hopes for a swift transfer of power in order to sign a security pact allowing some U.S. forces to remain in the country.

Berman however said that a deeper investigation into alleged fraud should not cause unnecessary delays.

"I hope the IEC would be able to perform this," he said. "It would not take much more time, it would not lead to a delay in the inauguration of the next president. The timetable would be respected."